The invention relates generally to an elevator control and, in particular, to a speed regulating circuit for controlling the stopping of an elevator car at a floor.
One type of elevator system includes a car driven by a polyphase alternating current motor which is coupled to a drive sheave. A tachometer dynamo is also coupled to the drive sheave to generate a signal representing the car speed and the rotational speed of the motor. During the arrival at a floor, the motor speed is regulable by a control circuit which is responsive to a set point transmitter which is switched in at the beginning of the arrival phase. The set point transmitter includes an integrator, which integrates an actual speed value produced by the tachometer dynamo and is connected at the output side with a subtractor, which forms a travel distance signal proportional to the difference between the actual travel distance signal formed by the integrator and a travel distance corresponding to the arrival distance to the floor at which the car will stop.
An elevator control system with a set point value transmitter according to the above description is disclosed, for example, by Swiss Patent Application No. 550 736. When control circuits of that kind, which assure great stopping accuracy, are used in elevator systems with polyphase current motors, then it is advantageous to let the motor, during the time which precedes the arrival, run unregulated at constant speed. The regulation during this time could consist only of braking the motor for each elevator to its smallest steady rotational speed with correspondingly high losses. However, difficulties arise during the transition from the unregulated to the regulated phase, since great differences between the load-dependent actual speed value and the suddenly arising set point speed value on the onset of the regulation for the arrival can arise and make themselves unpleasantly noticeable to the user of the elevator in the form of more or less hard jerks.
A control system, which avoids the aforementioned disadvantage, is disclosed in the West German Patent Application No. 3 010 234. In this case, a voltage-dependent fading regulator is provided, which during a portion of the speed-distance curve, continuously changes the influence of two mutually independent regulating circuits in dependence on the tachometer output voltage. The one regulating circuit regulates the acceleration in dependence on time, whilst the other regulating circuit regulates the speed in dependence on the travel distance. At the beginning of the braking process, the braking/time regulation is almost exclusively in engagement. Its effectiveness is reduced continuously with decreasing speed and that of the speed/distance regulation is increased correspondingly so that the speed/distance regulation is practically exclusively in engagement at the end of the braking phase. Thereby, a jerk-free transition is attained at the onset of the braking phase and as well as an exact arrival at the floor. The above-described control circuit is relatively complicated, since apart from two braking/time regulators and one speed/distance regulator, a fading regulator including at least five operational amplifiers must be provided.
The disadvantage of the known control circuits is based on the difference, which arises on the transition from an unregulated phase of the car travel to the regulated arrival phase, between the actual speed value and the set point speed value which is generated when merely a speed-regulating circuit is effective during the entire arrival phase.